Magnetically throttled liquefier assembly

ABSTRACT

A magnetically throttled liquefier assembly for use in an additive manufacturing system and configured to heat a metal-based alloy to an extrudable state includes an array of magnets to generate a magnetic field in order to induce a viscosity in the heated metal-based alloy and to control the flow rate of the heated metal-based alloy through the liquefier for extrusion and the building of a three-dimensional object with the metal-based alloy.

BACKGROUND

The present disclosure relates to additive manufacturing systems forbuilding three-dimensional (3D) parts with layer-based, additivemanufacturing techniques. In particular, the present disclosure relatesto extrusion-based additive manufacturing systems for building 3Dobjects with metal-based alloys.

An extrusion-based additive manufacturing system (e.g., fused depositionmodeling systems developed by Stratasys, Inc., Eden Prairie, Minn.) isused to build a 3D object from a computer-aided design (CAD) model in alayer-by-layer manner by extruding a flowable modeling material. Themodeling material is extruded through an extrusion tip carried by anextrusion head, and is deposited as a sequence of roads on a substrate.The extruded modeling material fuses to previously deposited modelingmaterial, and solidifies upon a drop in temperature. The position of theextrusion head relative to the substrate is incremented to deposit anext layer of material, and the process is then repeated to form a 3Dobject resembling the CAD model.

Movement of the extrusion head with respect to the substrate isperformed under computer control, in accordance with build data thatrepresents the 3D object. The build data is obtained from software thatinitially slices the CAD model of the 3D object into multiple slicedlayers, and generates tool paths for depositing roads of material withineach layer. Typically, the CAD model is sliced by an off-board computer,and tool paths instructions are transmitted to the additivemanufacturing system to define the build data for printing of the 3Dobject.

In fabricating 3D objects by depositing layers of modeling material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of objects under construction, whichare not supported by the modeling material itself. A support structuremay be built utilizing the same techniques used to print the 3D object.Additional geometry is generated to create a support structure for theoverhanging or free-space segments of the 3D object being formed. Thesupport structure is built up layer-by-layer in coordination withprinting of the 3D model, and may be built from a support material whichis deposited from a second extrusion, the support structure adheres tothe model during fabrication, and is removable from the completed 3Dobject when the build process is complete.

A common interest of consumers in the industry of additive manufacturingis to increase the physical properties of the 3D objects, such as partstrengths and durability. One category of materials that could providesuch increased physical properties include metal-based alloys. Forexample, 3D objects built from high-strength metals may exhibit tensilestrengths that are substantially greater than those of industrialthermoplastic materials. However, the extrusion of metal-based alloysposes several issues for additive manufacturing. For example, theextrusion of metal-based alloys requires high operating temperatures,which may undesirably affect performance of current digitalmanufacturing systems. Furthermore, heating a metal-based alloy to atemperature above its liquidus temperature may prevent the alloy fromhaving a sufficient viscosity for extrusion, and may undesirably affectits grain structure upon re-solidification (e.g., dendrite formation).Thus, there is an ongoing need for systems and methods for build 3Dobjects from metal-based alloys with additive manufacturing techniques.

SUMMARY

An aspect of the present disclosure relates to a magnetically throttledliquefier for use in an additive manufacturing system. The magneticallythrottled liquefier allows for controlled extrusion rates of a heatedmetal-based alloy to the same or similar degree with which othermaterials used in building 3D objects can be controlled (e.g.thermoplastic materials). The magnetically throttled liquefier isconfigured in one embodiment to induce an adjusted (e.g., increased)viscosity in a heated metal-based alloy to allow for flow control asliquidus metal-based alloy or freezing range metal-based alloy isdeposited to build the 3D object.

The magnetically throttled liquefier comprises in one aspect a liquefierconfigured to heat a feedstock of metal-based alloy to a flowable stateand a flow channel for receiving the heated metal-based alloy. The flowchannel is positioned within the liquefier such that a flow path of themetal-based alloy is perpendicular to a magnetic field, or to aplurality of spaced apart and alternating or oscillating magneticfields, generated by a magnet, or an array of magnets, that is, or are,also positioned within the liquefier. The magnetic field(s) are thendirected orthogonal to a flow direction through the flow channel alongthe length thereof. Motion of the heated metal-based alloy through thechannel orthogonal to the magnetic field(s) creates eddy currents in themetal-based alloy, allowing control of viscosity of the metal-basedalloy.

In another aspect, the liquefier includes a current source to pass acurrent through the flow channel orthogonal to the flow direction and tothe magnetic field(s) for even more control of the viscosity, allowingits use as a pump and a throttle for the heated metal-based alloy. Inthis embodiment, the interaction between the current, the magneticfield, and the moving metal-based alloy causes the induction of eddycurrents in the heated metal-based alloy. As the heated metal-basedalloy flows through the liquefier while subjected to an electric currentand a magnetic field or fields, the induced eddy currents in themetal-based alloy allow adjustment of a viscosity in the metal-basedalloy to make it suitable for controlled extrusion as a result ofresistive forces generated in the flow channel by the magnetic field,the current, the induced eddy currents and the flow or velocity of theheated metal-based allow. The magnetic field created by the eddycurrents resists relative motion of the heated metal-based alloy flowingin the channel and thus results in a magnetically throttled liquefier toeffectively control the flow rate of the heated metal based-alloythrough the flow channel and liquefier for extrusion.

Another aspect of the present disclosure is directed to a method forprinting a 3D part with an additive manufacturing system. The methodincludes feeding a metal-based alloy stock (e.g., a filament), to amagnetically throttled liquefier retained by the additive manufacturingsystem with a drive mechanism retained by the additive manufacturingsystem, and melting the metal-based alloy stock in the liquefier toproduce a liquidus or freezing range material. The method also includesproviding the heated metal-based alloy to a flow channel and utilizing amagnetic field, or alternating or oscillating magnetic fields, andelectrical current orthogonal to the magnetic field lines and thedirection of flow of the material in the flow channel to induce aselected viscosity in the heated metal-based alloy for regulating a flowand extrusion of the metal-based alloy in a controlled manner such thata 3D object can be formed.

DEFINITIONS

Unless otherwise specified, the following terms as used herein have themeanings provided below:

The terms “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the present disclosure.

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

The term “elevated temperature” as used herein with respect to a buildchamber desirably in the range from the solidification temperature ofthe metal-based alloy to the creep relaxation temperature of themetal-based alloy. Examples of suitable elevated temperatures for thebuild chamber range from about 200° C. to about 800° C., withparticularly suitable temperatures ranging from about 400° C. to about700° C., and with even more particularly suitable temperatures rangingfrom about 500° C. to about 650° C.

The terms “throttled” and “throttle” as used herein refer to the controlvia magnetic/electrical forces of flow of a heated metal-based alloywithin the embodiments of the disclosure, without using valves. Athrottled flow may have its flow rate increased or decreased in adirection of flow when the metal-based alloy is extruded and may incertain circumstances reverse the direction of flow to evacuate anextrusion tip using the magnetic/electrical throttle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an additive manufacturing system for building3D objects with metal-based alloys.

FIG. 2 is an expanded partial sectional view of an extrusion line of theextrusion head for extruding a metal-based alloy.

FIG. 3 is a further expanded partial sectional view of the extrusionline having a magnetic throttle.

FIG. 4 is an expanded partial side view of a flow channel component ofthe extrusion line of the extrusion head for extruding a metal-basedalloy.

FIG. 5 is an expanded partial top sectional view of an extrusion lineand flow channel component for extruding a metal-based alloy.

DETAILED DESCRIPTION

The present disclosure is directed to a magnetically throttled liquefierassembly having a flow channel configured for inducing viscosity in aheated metal-based alloy for controlling the flow of the metal-basedalloy for extrusion. The magnetically throttled liquefier may beconfigured for use in an additive manufacturing system, such as anextrusion-based additive manufacturing system. The liquefier assemblyincludes an extrusion line having a flow channel configured to induce aviscosity in a heated metal-based alloy that is suitable for extrusionand allows for flow control of the metal-based alloy through theliquefier to an extrusion tip. As discussed below, the liquefiercomprises in one embodiment a flow channel that is configured to directthe flow of heated metal-based alloy through the channel in response toa magnetic field and an electrical current passed through the heatedmetal-based alloy in the flow channel to induce an increased viscositysuch that the heated metal-based alloy may be extruded in a controlledmanner.

FIG. 1 is a front view of an embodiment of an additive manufacturingsystem 10, which is preferably a high-temperature, digital manufacturingsystem for building 3D objects with metal-based alloys. As illustrated,system 10 includes housing 12 (shown with broken lines), controller 13,build chamber 14, platform assembly 16, head assembly 18, and quenchtank 20. Housing 12 is the exterior cover of system 10, which protectsthe internal components of system 10 from external conditions. System 10may also include support frames (not shown) for retaining build chamber14, platform assembly 16, and head assembly 18 within housing 12 at therespective locations shown in FIG. 1.

Build chamber 14 may be an enclosed, high-temperature environment inwhich 3D objects (represented as 3D object 22 in FIG. 1) are built withone or more metal-based alloys. Build chamber 14 desirably functions asa high-temperature oven, and is desirably maintained at one or moreelevated temperatures to reduce the risk of mechanically distorting(e.g., curling) 3D object 22, and to decrease shrinkage due to theeffects related to the coefficient of thermal expansion of themetal-based alloy. The temperature of build chamber 14 may be elevatedthrough the use of electrical and/or flame-based mechanisms using, forexample, timed thermal ramping cycles.

The system 10 comprises a platform assembly 16, which includes drivemotor 28, z-axis gantry 30, and build platform 32 and head assembly 18which includes drive motors 34 and 36, x-y-axis gantry 38, and extrusionhead 40. Suitable examples of system 10 are described in Crump et al.U.S. Pat. No. 7,942,987, the disclosure of which is hereby incorporatedby reference in its entirety.

During a build operation, build chamber 14 may be substantially purgedof oxidizing gases (e.g., purging with argon and/or vacuum), and heatedto one or more elevated temperatures. Controller 13 then directs drivemotors 34 and 36 to move extrusion head 40 around within build chamber14 in the horizontal X-Y plane via x-y-axis gantry 38. Controller 13also directs extrusion head 40 to extrude the metal-based alloy ontobuild platform 32 in a pattern based on the movement of extrusion head40, thereby forming a layer of 3D object 22.

When most metal-based alloys are heated to a temperature at or abovetheir respective liquidus temperature, the metal-based alloys have a lowviscosity, often lower than water. These heated metal-based alloys alsopossess a high surface tension in addition to a tendency to re-melt asurface on which the heated metal-based alloy is applied. The propertiesof heated metal-based alloys have made building 3D objects withextrusion-based digital manufacturing systems difficult to control.

Prior art approaches to controlling the flow of heated metal-basedalloys for extrusion and deposition have included raising the viscosityof the heated metal-based alloy by incorporating solid particles intothe melt to increase the effective viscosity. Implementing a slush-phase(peritectic) metal-based alloy is an artificial means to increase theeffective viscosity of alloys for extrusion based digital manufacturing,however this means can result in clogging of the extrusion line.

Advantageously, embodiments of the present disclosure are directed toincreasing the viscosity of heated metal-based alloys for extrusion. Thepresent disclosure is directed to an embodiment capable of increasingthe viscosity of a liquidus metal-based alloy, with or withoutimplementing a slush-phase, and without fully melting a previouslyextruded layer, while allowing the heated metal-based alloy to reliablyflow through the extrusion line to an extrusion tip without cloggingresulting from the crystal population of the slush phase. In someembodiments, the need for a filter or other mechanism to remove orprevent crystal population of the heated metal-based alloy may beeliminated.

Metal alloys, particularly aluminum alloys, have high electricalconductivity. The present disclosure uses the effect of passing aconductive element, the heated metal-based alloy, through a magneticfield which induces an increased viscosity in the heated metal-basedalloy. By passing an induced electrical current through the movingconductive element a second magnetic field, an induced magnetic field,is generated. As the induced magnetic field is in such a direction as tooppose the motion or change that caused it, the induced magnetic fieldis oriented in a direction that opposes the flow direction of the heatedmetal-based alloy, allowing the magnetically throttled liquefier toinduce a viscosity in the heated metal-based alloy and to control theflow of the heated metal-based alloy through the extrusion line. In anembodiment of the liquefier according to the present disclosure, theflow of the heated metal-based alloy through the liquefier can beeffectively controlled by the magnetically throttled liquefier.

A modest magnetic field, for example, of one Tesla, passing through anextrusion tip pipe of a metal-based alloy extrusion system can beestimated to increase the effective viscosity of the heated metal-basedalloy by roughly a factor of 3,000. The magnetically-induced viscositycan be high enough such that the viscosity outweighs effects from otherforces present in the system, including surface tension, gravity, partback pressure, and inertia on the heated metal-based alloy. Thus, amagnetic field can be incorporated in one embodiment into the extrusionline to control the flow of the heated metal-based alloy. The magneticfield can be used in connection with a current flow through themetal-based alloy to effectively control the flow of the heatedmetal-based alloy through the liquefier orthogonal to the current and tothe magnetic field. The flow of the heated metal-based alloy is then ascontrollable as that of other materials used in additive manufacturingsystems, for example, amorphous plastics.

FIG. 2 illustrates an expanded partial sectional view of extrusion line112 of extrusion head 40, also referred to as a print head, (illustratedin FIG. 1) for extruding, or printing, a metal-based alloy to build 3Dobject 22 (illustrated in FIG. 1). Extrusion line 112 includes feed tube114, coolant assembly 116, drive mechanism 118, liquefier assembly 120,and extrusion tip 122. In one embodiment, feed tube 114 receives themetal-based alloy in a wire form (represented as wire 124) from a supplysource of wire 124 located external to build chamber 14, where wire 124is supplied to extrusion head 40 through an umbilical (not shown). Thedimensions of wire 124 may vary depending on the metal-based alloy used,and on the dimensions and capabilities of feed tube 114, drive mechanism118, and liquefier assembly 120. Examples of suitable average diametersfor wire 124 range from about 0.508 millimeters (about 0.020 inches) toabout 2.54 millimeters (about 0.100 inches). In embodiments in whichwire 124 is substantially rigid due to its diameter, the radius ofcurvature of feed tube 114 (represented as radius 126) is desirably atleast fifty times the diameter of wire 124 to reduce friction withinfeed tube 114.

The extrusion line 112 includes a liquefier assembly 120 having aliquefier tube 136 and liquefier block 138. In another embodiment,heater assemblies 158 are used to provide more control of temperature ofthe heated metal-based allow in the liquefier tube 136. The liquefiertube 136 is, in one embodiment, a thin-walled, thermally conductive tubeextending through liquefier block 138, and has an entrance proximatedrive mechanism 118, and an exit at extrusion tip 122.

Liquefier block 138 is a heating block for heating wire 124 to atemperature suitable for a desired flow pattern based on a thermalprofile along liquefier block 138. Due to the high thermal conductivityof the metal-based alloy (relative to thermoplastic materials), thelength of thermal profile along liquefier block 138 may be reduced,which correspondingly reduces the flow response time during the buildoperation. Extrusion tip 122 is secured to a distal end of liquefierassembly 120, and has a tip diameter for depositing roads of themetal-based alloy, where the road widths and heights are based in parton the tip diameter.

An extrudable state is reached by heating the metal-based alloy to anelevated temperature, the elevated temperature being at least between asolidus state and a liquidus state of the metal-based alloy. At least aportion of the heated metal-based alloy may be heated to an elevatedtemperature at or above a liquidus state of the metal based alloy. Theliquefier 120 is configured to heat the metal-based alloy to anextrudable state and is further configured to induce a viscositysuitable for controlled extrusion in the heated metal-based alloy.

Heating the wire 124 may be performed in a number of different wayswithout departing from the scope of the disclosure. For example, inaddition to the heating methods and apparatus discussed herein, magneticinduction may be used as a heat source for converting a wire input toliquid. In one embodiment, a magnetic induction heater may be used inthe extrusion head 40 without departing from the scope of thedisclosure.

Extrusion line 112 utilizes magnetohydrodynamic drive to control theflow of the heated metal-based alloy. As illustrated in further detailin FIGS. 3-4, liquefier tube 136 comprises channel 140 having anentrance along a length of liquefier tube 136 above extrusion tip 122and an exit near extrusion tip 122, which provides a flow channel forheated metal-based alloy in a first direction, for example thez-direction. An array of magnets 142 is positioned in liquefier block138 and extending along a length of liquefier tube 136 at least to theextent of the length of channel 140. The array of magnets 142 comprisesone or more magnets, which for example, may be permanent magnets such asneodymium magnets able to withstand elevated temperatures in liquefierblock 138. The array of magnets produces oscillating or alternatingmagnetic fields oriented in a second direction, the x-direction asillustrated by arrows 143. Thus, each of the magnetic field lines issubstantially perpendicular to flow channel 140 and to the direction offlow of the heated metal-based alloy in flow channel 140. The movementof the heated metal-based alloy through the channel and orthogonal tothe magnetic field induces eddy currents in the alloy. These eddycurrents create a magnetic field that opposes the magnetic field throughwhich the heated metal-based alloy passes, increasing its viscosity, andtherefore reducing the flow rate. This allows for throttling of theflow.

In such a magnetohydrodynamic drive, the pump configuration may be usedto generate a small backward flow of heated metal-based alloy, such as,for example, a roll-back to evacuate the extrusion tip 122. Whilediscussed as a throttle, it should be understood that the embodiments ofthe present disclosure may be used as a stand-alone pump as well, suchas in pumping sodium-potassium alloys (NaK) in nuclear breeder reactors,for example.

While FIG. 3 shows magnets 142 on alternating sides of the liquefiertube 136, it should be understood that placement of magnets may bevaried while still retaining alternating magnetic fields, withoutdeparting from the scope of the disclosure. Further, magnets may bepermanent magnets, or may be electromagnets or other magnetic fieldproducing elements, also without departing from the scope of thedisclosure.

Channel 140 may be further configured with a current source 144 toprovide a current along the y-direction orthogonal to flow channel 140and to magnetic field lines, and to pass current through flow channel140 to the heated metal-based alloy in channel 140. Current source 144may in one embodiment comprise terminals (e.g., electrodes) positionedin contact with an outer surface of channel 140 such that the electrodesare configured to pass current into channel 140. Alternatively,electrodes 144 may actually be formed within channel 140. Formingelectrodes 144 directly in channel 140 may eliminate the need forelectrical contacts between electrodes 144 and a source of current whenmultiple magnets 142 are positioned with alternating poles along channel140 and where channel 140 is configured for annular flow. The currentand magnetic fields combined with eddy currents created in the alloyform an electromagnetic pump with controllable extrusion of the heatedmetal-based alloy, as is described further herein. The pump may be usedto throttle flow as well as to increase flow in the flow channel.Current source 144 in one embodiment may be an annular current thatcreates a variable magnetic field depending on the magnitude of thecurrent, allowing further control of the eddy currents generated in theheated metal-based alloy.

Referring also to FIG. 4, the magnetic field produced by magnets 142along with the flow of the heated metal-based alloy induces eddycurrents within the heated metal-based alloy. The induced currentrepresented by arrows 145 induces a second magnetic field which opposesthe direction of the supplied magnetic field(s) 143. Thus, relativemotion between the magnetic field and the flowing heated metal-basedalloy induce eddy currents 145 that in turn create a magnetic fieldresistive to the direction of flow of the heated metal-based alloy. Thisresistive force acts as a back pressure which effectively induces aviscosity in the metal-based alloy and allows for flow control includingbraking and pumping of the flow of the heated metal-based alloy throughchannel 140 by manipulation of current through the channel 140.

Electrodes 144 positioned in channel 140 are configured to pass currentthrough flow channel 140 and into the heated metal-based alloy, inducingeddy currents 145 within the alloy. As illustrated in FIG. 4, circularcurrents are generated along the length of channel 140 in the heatedmetal-based alloy flowing therethrough. Channel 140 and the heatedmetal-based alloy act as a conductor where the flow of heatedmetal-based alloy acts as a moving conductor. The induced eddy currentsgenerate second magnetic fields, the induced magnetic fields, whichoppose the supplied magnetic field(s) and react back on the movingconductor. The circular currents generate a magnetic field that createsa force resisting the metal-based alloy flow in the presence of themagnetic field 143. Referring to the embodiment illustrated in FIGS.4-5, five magnet and yoke assemblies may be positioned along the lengthof channel 140 in order to produce the magnetic fields and electricfields illustrated in FIG. 4.

FIG. 5 is an expanded top cross-sectional view of an embodiment ofchannel 140. Flow channel 140 may be positioned in liquefier block 138and surrounded by a softened metal, for example, soft iron 156.Incorporating a softened metal into liquefier block for surroundingchannel 140 allows the field lines from permanent magnets 142 to bedirected perpendicularly through heating elements 158 of liquefier tube136 and thus perpendicularly through flow channel 140. The soft iron 156also reinforces channel 140 to counter act pressure buildup in channel140 due to the magnetic and electrical fields increasing the pressure onthe heated metal-based alloy, preventing bowing of channel 140.

In one embodiment, heating elements 158 and a temperature sensor element160 may be positioned in the liquefier block 138 allowing formaintenance of the elevated temperature of the heated metal-based alloywhile monitoring the temperature of the heat resistant components of theliquefier assembly. Heating elements 158 may comprise thick film heatersor HEATRON® plates, which surround liquefier tube 136 and a hightemperature stability fitting or gasket 162. Gasket 162 is configured tomaintain a seal in the system 10 where elevated temperatures arecontinuously maintained for heating the metal-based alloy. In theillustrated embodiment, a gasket able to maintain a seal at continuoustemperatures as high as 1000° C. is used. Magnets 142 also may comprisea magnet and yoke assembly, where the magnetic material, withoutwindings, permanently connects two or more magnet cores.

A spacer 164 is positioned in one embodiment between each magnet 142 andthe liquefier tube 136. This spacer, combined with the soft metalportions 156, allow the direction of the magnetic field lines to beoriented normal to the metal-based alloy flow direction.

The viscosity induced in the heated metal-based alloy can be estimatedbased on various system arrangements. In the embodiment illustrated inFIGS. 4-5, magnetic field 143 is produced and directed through thickness146 of channel 140 by the array of permanent magnets 142 arranged withalternating poles extending along the length 148 of channel 140 as shownin FIG. 3. For the annular flow channel 140, the temperature independentmagnetic field strength B(y) may be determined pursuant to Equation 1:

B (y)=B _(o) sin(ky){circumflex over (y)}

where the length of each one of the loops may be determined pursuant toEquation 2:

L _(loop)=2w+π/k

An alloy flow with velocity ν will generate eddy current loops in thedirection of arrows 145 where the electromotive force (emf) around oneof the loops may be determined pursuant to Equations 3 and 4:

${emf} = {\frac{}{t}{{\overset{\_}{B}(y)} \cdot \overset{\_}{A}}\text{;}\mspace{14mu} {and}}$${emf} \cong {B_{o}{wv}\frac{\pi}{4}}$

Where the conductivity ρ of the alloy is taken, for example, to beapproximately 2.5×10⁷ mho/m and the current loop length is as determinedby Equation 2, a current I_(loop) for each of the N number of loops isshown in Equation 5:

$I_{loop} = \frac{B_{o}{wv}\; \pi^{2}h\; \rho}{\pi + {2\; {wk}}}$

where the force F_(loop) on the alloy resisting the velocity ν may bedetermined pursuant to Equation 6:

F _(loop) =I _(loop) wB _(o)

and the pressure P_(B) resisting the flow produced by all N loops may bedetermined pursuant to Equation 7:

$P_{B} = {\frac{B_{o}^{2}{wv}\; \pi^{2}\; \rho \; N}{\pi + {2\; {wk}}}.}$

Taking the length L of the channel to be N π/2k, the pressure P_(B) maybe determined pursuant to Equation 8:

$P_{B} = {\frac{2B_{o}^{2}w\; \pi \; \rho \; k}{\pi + {2\; {wk}}}L\; v}$

The pressure drop P_(η) generated by the same channel produced by anaverage velocity ν and a viscosity η may be determined pursuant toEquation 9:

$P_{\eta} = {\frac{12\; \eta}{h^{2}}L\; v}$

The braking pressure in each of Equations 8 and 9 is proportional to thealloy velocity ν and to the channel length L. The magnetic viscosityη_(B) induced by eddy currents may be determined pursuant to Equation10:

$\eta_{B} = \frac{B_{o}^{2}h^{2}w\; \pi \; \rho \; k}{6\left( {\pi + {2\; {wk}}} \right)}$

In a case in which a spacing of the field sign changes is set to k=π/w,the magnetic viscosity η_(B) induced may be determined pursuant toEquation 11:

$\eta_{B} = \frac{B_{o}^{2}h^{2}\; \pi \; \rho}{18}$

As shown in Equations 10 and 11, increasing the and increasing themagnetic field correspondingly increases the magnetically inducedviscosity of the heated metal-based alloy with the square of the gap andthe square of the magnetic field, thereby increasing the controllabilityof the metal-based alloy flow through extrusion line 112. Additionally,as shown in the Equations above, metals and metal-based alloys having ahigher conductivity illustrate ability for greater increase inmagnetically induced viscosity.

The magnetically throttled liquefier 120 allows the flow rate of heatedmetal-based alloy through extrusion line 112 to be controlled by thecontroller 13 by manipulating the magnetic field and/or manipulating thecurrent created in by the current source. The flow control isindependent of the temperature of the heated metal-based alloy andindependently of the heat capacity of the material directly underextrusion tip 122, as these parameters can vary widely depending onvarious features of additive manufacturing system 10. In someembodiments, adjusting the extrusion tip temperature and the draw (e.g.gap or spacing between the extrusion tip and the 3D part or lastdeposited layer) can allow for control of the heated metal-based alloyflow for start-up and shut-off of extrusion, allowing for starting upand shutting off of roads in the process of building the 3D part.Controlling the extrusion of the heated metal-based alloy duringdeposition may be accomplished by manipulating one or both of theelectric field and magnetic field around the heated metal-based alloyand an electric current directed through the heated metal-based alloy inthe flow channel.

The present disclosure uses the concept that a viscosity of a heatedmetal-based alloy, or liquidus portion of the heated metal-based alloy,can be adjusted, providing the heated metal-based alloy with a viscositysuitable for extruding the metal-based alloy to build a 3D object. Theheated metal-based alloy may include a combination of liquidusmetal-based alloy and heated metal-based alloy heated to an elevatedtemperature between a solidus state of the metal-based alloy and aliquidus state of the metal-based alloy. The viscosity of the heatedmetal-based alloy may be further controlled by altering the strength ofthe magnetic field and the flow rate of the heated metal based alloy maybe controlled by adjusting the current applied to the heated metal-basedalloy in the flow channel. Accordingly, the metal-based alloy may beheated in liquefier block 138 to at least a partially liquidus state anda viscosity induced such that the heated metal-based alloy is suitablefor extrusion from extrusion tip 122. Examples of suitable viscositiesfor extruding the metal-based alloys range from about 1 poise to about1,000 poise, with particularly suitable viscosities ranging from about 5poise to about 500 poise, and with even more particularly suitableviscosities ranging from about 10 poise to about 100 poise.

Accordingly, suitable metal-based alloys for use with system 10 includeany alloy containing two or more metal elements and that exhibits atleast some electrical conductivity in a liquidus state. Examples ofsuitable metal-based alloys include aluminum based alloys, which havehigh electrical conductivity, for example aluminum alloys includingabout 90% by weight to about 95% by weight aluminum Such alloys exhibitrelatively low liquidus temperatures, and have sufficient conductivityfor viscosity control.

While specific alloys have been discussed, it should be understood thatadditional metals and alloys thereof may be employed in the variousembodiments without departing from the scope of the disclosure. Forexample, magnetic throttling may also be performed using, for exampleonly and not by way of limitations, silver, copper, gold, rhodium,indium, ruthenium, osmium, palladium, and platinum, lead, tin and anycombinations thereof.

Examples

The present disclosure is more particularly described in the followingexample that is intended as an illustration only, since numerousmodifications and variations within the scope of the present inventionwill be apparent to those skilled in the art. Unless otherwise noted,all parts, percentages, and ratios reported in the following examplesare on a weight basis, and all reagents used in the examples wereobtained, or are available, from the chemical suppliers described below,or may be synthesized by conventional techniques.

A heated metal-based alloy having a viscosity (η) of 0.5 centipoise willbe directed through a channel having a length of approximately two (2)inches, a width of approximately 0.25 inches, and thickness ofapproximately 0.04 inches at a velocity (ν) of 10 inches per second.Along the length of the channel there are seven alternating poles(magnets) per inch and nine current loops along the length of thechannel Taking Q=1000 mic/s and the initial magnetic field (B_(o)) at 1Tesla, the pressure drop from the viscosity of the alloy is determinedto be P_(η)=0.011 psi, and the pressure drop from the current drag isdetermined to be 77.8 psi. The magnetic field determined above resultsin a magnetically induced viscosity of the heated metal-based alloy ofapproximately 45 poise, or 7,000 times the normal viscosity of theheated metal-based alloy.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed:
 1. A liquefier assembly for use in an additivemanufacturing system to print three-dimensional parts, the liquefierassembly comprising: a liquefier tube extending through a liquefier andconfigured to receive and heat a feedstock of a metal-based alloy to anelevated temperature; at least one magnet positioned in the liquefierassembly and configured to produce a magnetic field in the liquefierassembly; a flow channel in fluid communication with the liquefier tubeand an extrusion tip and positioned within the magnetic field generatedby the at least one magnet and configured to magnetically induce anincreased viscosity in the heated metal-based alloy flowingtherethrough; and wherein the extrusion tip is configured to receive anddeposit the heated metal-based alloy having the magnetically inducedviscosity.
 2. The liquefier assembly of claim 1, wherein the at leastone magnet comprises an array of magnets positioned along a length ofthe flow channel assembly wherein the magnets are arranged to providemagnetic fields in alternating orientations.
 3. The liquefier assemblyof claim 2, wherein the array of magnets is oriented to produce magneticfields in a direction substantially perpendicular to a flow direction ofthe heated metal-based alloy in the flow channel.
 4. The liquefierassembly of claim 1, and further comprising a current source configuredto pass an electrical current through the flow channel assembly to theheated metal-based alloy in the flow channel.
 5. The liquefier assemblyof claim 4, wherein the current source is configured to apply a currentto the flow channel in a direction orthogonal to flow of the heatedmetal-based alloy in the flow channel and to the magnetic field.
 6. Theliquefier assembly of claim 5, wherein the flow channel is configured toreceive an electrode of the current source therein.
 7. The liquefierassembly of claim 1, wherein the metal-based alloy is an aluminum basedalloy comprising about 90% by weight to 95% by weight aluminum.
 8. Theliquefier assembly of claim 3, wherein a viscosity is induced in theheated metal-based alloy and the induced viscosity can be estimatedaccording to the following equation:$\eta_{B} = {\frac{B_{o}^{2}h^{2}w\; \pi \; \rho \; k}{6\left( {\pi + {2\; {wk}}} \right)}.}$9. The liquefier assembly of claim 1, wherein a softened metal surroundsthe flow channel and the array of magnets and is configured to reinforcethe flow channel against pressure build-up in the flow channel and todirect the magnetic field lines perpendicularly through the flowchannel.
 10. A method for building a three-dimensional object in alayer-by-layer manner with an additive manufacturing system comprising:heating a build chamber of the additive manufacturing system to aselected elevated temperature; heating a metal-based alloy in anextrusion line of the additive manufacturing system, wherein themetal-based alloy is heated to an elevated temperature such that themetal-based alloy is in a flowable state; generating a first magneticfield along a portion of a length of the extrusion line, the magneticfield lines being perpendicular to a flow direction of the extrusionline; increasing the viscosity of the heated metal-based alloy byflowing the heated metal-based alloy through a flow channel positionedwithin the magnetic field; depositing the heated metal-based alloyhaving the increased viscosity from a deposition head of the additivemanufacturing system onto a build platform of the additive manufacturingsystem in a predetermined pattern; and solidifying the heatedmetal-based alloy to provide a re-solidified alloy.
 11. The method ofclaim 10, wherein the extrusion line is disposed outside of the buildchamber and the deposition head is disposed within the build chamber,and wherein the method further comprises driving the heated metal-basedalloy from the extrusion line to the deposition head through themagnetic field to induce a viscosity in the heated metal-based alloy.12. The method of claim 10, wherein the metal-based alloy comprises ametal having electrical conductivity.
 13. The method of claim 12,wherein the metal-based alloy is an aluminum based alloy comprisingabout 90% by weight to 95% by weight aluminum.
 14. The method of claim10, and further comprising passing a current through the heatedmetal-based alloy flowing through the flow channel to regulate the flowof the heated metal-based alloy through the extrusion line.
 15. Themethod of claim 14 and further comprising passing the electrical currentto the heated metal-based alloy flowing through the flow channel with atleast one electrode positioned within the channel.
 16. The method ofclaim 15 and further controlling the flow rate of the heated metal-basedalloy through the flow channel for extrusion based on the relationshipof the magnetic field strength and the electrical current passed to theheated metal-based alloy as the current generates a second magneticfield opposing the direction of the first magnetic field and applies apressure to the heated metal-based alloy flow in the flow channel alongan axis of the flow direction.
 17. The method of claim 10, wherein themagnetically induced viscosity can be estimated according to thefollowing equation:$\eta_{B} = {\frac{B_{o}^{2}h^{2}w\; \pi \; \rho \; k}{6\left( {\pi + {2\; {wk}}} \right)}.}$18. A liquefier assembly for use in an additive manufacturing system toprint three-dimensional parts, the liquefier assembly comprising: aliquefier tube having an inlet end and an outlet end and configured toreceive a feedstock of a metal-based alloy; a heat source positionedproximate the liquifier tube and configured to heat the metal-basedalloy feedstock to temperature at which the metal-based alloy becomesliquidus; an extrusion tip secured to the outlet end of the liquefiertube wherein the extrusion tip is configured to deposit the heatedmetal-based alloy; a flow controller in communication with the liquifiertube, the flow controller comprising: at least one magnet positioned inthe liquefier assembly and configured to produce a magnetic field in theliquefier assembly and configured to induce an increased viscosity inthe heated metal-based alloy flowing therethrough wherein a strength ofthe magnetic field can be varied to manipulate a flow rate of the heatedmetal-based alloy; and a current source configured to pass a currentthrough the liquifier tube in a direction substantially orthogonal to adirection of flow of the heated metal-based alloy and to the magneticfield and wherein the current source is configured to change a currentpassed through the liquifier tube to manipulate the flow rate of theheated metal-based alloy.
 19. The liquefier assembly of claim 18,wherein the at least one magnet comprises an array of magnets positionedalong a length of the flow channel assembly wherein the magnets arearranged to provide magnetic fields in alternating orientations.
 20. Theliquefier assembly of claim 19, wherein the array of magnets is orientedto produce magnetic fields in a direction substantially perpendicular toa flow direction of the heated metal-based alloy in the flow channel.